In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water (fountain solution) in the process of printing. Lithographic printing is a printing method utilizing the nature of dampening water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate-making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image area of the image-recording layer, removing the image-recording layer in other unnecessary areas by dissolving with an alkaline developer or an organic solvent thereby revealing the hydrophilic surface of support.
In the hitherto known plate-making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary image-recording layer by dissolving, for example, with a developer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate-making methods in response to the above-described requirement, a method referred to as on-machine development has been proposed wherein a lithographic printing plate precursor having an image-recording layer capable of being removed in the unnecessary areas during a conventional printing process is used and after exposure, the non-image area of the image-recording layer is removed on a printing machine to prepare a lithographic printing plate.
Specific methods of the on-machine development include, for example, a method of using a lithographic printing plate precursor having an image-recording layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image-recording layer by contact with rollers or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image-recording layer or adhesion between an image-recording layer and a support upon penetration of dampening water, ink solvent or the like and then mechanically removing the image-recording layer by contact with rollers or a blanket cylinder of a printing machine.
In the invention, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing an unexposed area in an image-recording layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, an alkaline developer) thereby revealing a hydrophilic surface of support. The term “on-machine development” means a method and a step of removing an unexposed area in an image-recording layer of the lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image-outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate technique of carrying digitalized image information on highly converging radiation, for example, a laser beam and conducting scanning exposure of a lithographic printing plate precursor with the radiation thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
As described above, in recent years, the simplification of plate-making operation and the realization of dry system and non-processing system have been further strongly required from both aspects of the consideration for global environment and the adaptation for digitization.
As such a lithographic printing plate precursor, for example, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer containing a hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder is known (refer to, for example, Japanese Patent 2,938,397). The lithographic printing plate precursor can be subjected to the on-press development by exposing with an infrared laser, fusing and coalescing the hydrophobic thermoplastic polymer particles by heat to form an image, loading on a cylinder of a printing machine, and supplying dampening water and/or ink.
However, such a method of forming an image by coalescence due to mere heat fusion of polymer fine particles certainly shows good on-machine developing property, but the image strength is extremely weak and printing durability is insufficient.
In order to improve a lithographic printing plate precursor capable of being subjected to on-machine development in the printing durability, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon a heat-sensitive layer containing microcapsules containing a compound having a functional group reactive with heat, wherein the heat-sensitive layer or a layer adjacent to the heat-sensitive layer contains an infrared absorber, is proposed (refer to JP-A 2001-277740 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2001-277742).
However, the technique is still insufficient in view of practical printing durability.
Also, as another technique for improving the printing durability, a lithographic printing plate precursor capable of being subjected to on-machine development which comprises a support having provided thereon a photosensitive layer containing an infrared absorber, a radical polymerization initiator and a polymerizable compound is known (refer to JP-A-2002-287334). Further, a lithographic printing plate precursor capable of being subjected to on-machine development after exposure which comprises an aluminum support provided thereon a photosensitive composition containing (a) a water-soluble or water-dispersible polymer, (b) a monomer or oligomer having a photopolymerizable ethylenically unsaturated double bond, and (c) a photopolymerization initiation system having an absorption maximum in an ultraviolet region is described in JP-A-2000-39711.
In these methods of using a reaction, for example, a polymerization reaction, the density of chemical bonding of the image area is high as compared with the image area formed by the heat fusion of polymer fine particles, therefore it is possible to increase image strength. However, they are still insufficient from the standpoint of satisfying both of the on-machine developing property and fine line reproducibility or printing durability, and further improvements have been desired.
On the other hand, in a lithographic printing plate precursor utilizing a radical polymerization reaction, a protective layer (an overcoat layer) is provide on an image-recording layer for the purpose of preventing the generation of scratches, blocking oxygen, preventing ablation by the exposure with a high illuminance laser beam or the like. Although a layer containing a water-soluble polymer compound as a main component has been heretofore used as the protective layer, such a layer is still insufficient for satisfying all of the on-machine developing property, fine line reproducibility, printing durability, ink-receptive property and on-machine development running property (aptitude, for example, mixing of the substances removed by the on-machine development in dampening water by continuously repeating the on-machine development and printing). Therefore, further improvements have been desired.